A Drupal Theme is a basically website design.
In Drupal, the design of a website is seperated from the data of a website.
For example, the text you are reading now is stored in a database. The "theme" makes it appear how it does - with a big orange title, and text on the left side of the page, and menus and further information on the right side of the page.
You can install a new theme and completely change the design of your Drupal website, without having to change your data. The data stays the same, but the website design changes.
Drupal Themes can be written for one website, and then installed on other Drupal websites. Just like programs written for your computer.
So in summary, a Drupal Theme governs the way a website looks (like font sizes and page layout).
